Embodiments relate generally to transport refrigeration systems, and more particularly to adaptive control of a multi-compartment transport refrigeration system.
The refrigerated container of a truck trailer requires a refrigeration unit for maintaining a desired temperature environment within the interior volume of the container. A wide variety of products, ranging for example, from freshly picked produce to deep frozen seafood, are commonly shipped in refrigerated truck trailers and other refrigerated freight containers. To facilitate shipment of a variety of products under different temperature conditions, some truck trailer containers are compartmentalized into two or more separate compartments, each of which typically having a door that opens directly to the exterior of the trailer. The container may be compartmentalized into a pair of side-by-side axially extending compartments, or into two or more back-to-back compartments, or a combination thereof.
Conventional transport refrigeration units used in connection with compartmentalized refrigerated containers of truck trailers include a refrigerant compressor, a condenser, a main evaporator and one or more remote evaporators connected via appropriate refrigerant lines in a closed refrigerant flow circuit. The refrigeration unit must have sufficient refrigeration capacity to maintain the product stored within the various compartments of the container at the particular desired compartment temperatures over a wide range of outdoor ambient temperatures and load conditions.
In addition to the afore-mentioned main evaporator, one or more remote evaporators, typically one for each additional compartment aft of the forward-most compartment, are provided to refrigerate the air or other gases within each of the separate aft compartments. The remote evaporators may be mounted to the ceiling of the respective compartments or mounted to one of the partition walls of the compartment, as desired. The remote evaporators are generally disposed in the refrigerant circulation circuit in parallel with the main evaporator.
Multiple temperature compartment transport refrigeration systems create significant control and refrigeration system complexity. Existing systems couple the main evaporator and remote evaporators to a common compressor suction plenum. When two or more compartments cool simultaneously in a system with a common suction plenum, the saturated evaporation temperature is shared between all compartments and evaporators. The resulting common evaporating temperature is dictated by coldest temperature compartment. Controls need to be put in place to prevent a perishable compartment from flooding or over feeding the frozen compartment. This is due to the fact that the perishable saturated evaporating temperature is significantly lower than perishable compartment air temperature. When saturation temperatures in the perishable compartment are low, the resulting sensed superheat for the perishable compartment is high. This causes the expansion device (e.g., mechanical or electrical) to open to 100%, which can result in flooding of the evaporator in the frozen compartment.
Existing systems employ pulsed cooling to prevent the perishable compartment capacity demand from flooding and diminishing all available capacity in the frozen or colder compartment. To combat this problem controls are put on a liquid solenoid valve to limit the available capacity to the perishable compartment. Typically a fixed pulse width modulation (PWM) cycle is used to control the liquid flow to the compartment that is trying to cool at the higher air temperature compartment when simultaneous cooling is required. This pulse width modulated approach and a high sensed superheat can cause a very dynamic power disturbance on the engine and introduce engine and control instability.